A remote lock/unlock control apparatus is disclosed in JP 2005-113608A as one example of a conventional vehicle device remote control system for a vehicle. This vehicle device remote control system is proposed to avoid mutual interference of request signals of a subject vehicle and other vehicle in a case that a portable unit of the subject vehicle is located in a zone, in which an effective transmission zone (referred to also as detection area) of the request signal of the subject vehicle and an effective transmission zone of the request signal of the other vehicle overlap. Specifically, the control apparatus mounted in the vehicle varies a periodic transmission interval Ti of the request signal at each transmission of the request signal.
However, this lock/unlock control apparatus only varies the periodic transmission interval Ti at each transmission of the request signal. The portable unit therefore is likely to continue transmission of a response signal in response to the request signal transmitted from the other vehicle before receiving the request signal transmitted from the subject vehicle. As a result, the portable unit consumes more power because it transmits the response signal unnecessarily.